In circuitry that includes sensitive components, for example charging circuitry of an electronic device, protection from over voltage may be provided through the use of a transient voltage suppressor. The transient voltage suppressor generally reduces the amount of voltage provided to sensitive circuitry components by diverting an amount of voltage to ground or away from the sensitive circuitry components. However, in a situation where the transient voltage suppressor has become disabled or disconnected from the circuitry, no protection from over voltage is provided to the sensitive components. This may result in damage to the sensitive components due to the increased amount of current flowing through the sensitive components as a result of the increased voltage. In addition, the increased voltage may also result in other unsafe conditions.
Institute of Electrical and Electronics Engineers (IEEE) 1725 standard provides that charging circuitry, in particular charging circuitry related to mobile telephones, must be safe when there are two simultaneous faults. IEEE 1725 establishes criteria for design analysis for qualification, quality, and reliability of rechargeable lithium ion and lithium polymer batteries for cell phone applications. The purpose of IEEE 1725 is to ensure reliable user experience and operation of cell phone batteries.
For example, the voltage of a charging power source provided to the charging circuitry may suddenly increase due to a spike in voltage of an electrical distribution system. In this instance, the voltage provided to the charging circuitry is too high, and the transient voltage suppressor is needed to direct the over voltage away from the charging circuitry. In certain circumstances the transient voltage suppressor may not be able to direct over voltage away from the charging circuitry. For example, the transient voltage suppressor may heat up due to the increased current passing through it, and become disconnected from the charging circuitry. In this case, the transient voltage suppressor will no longer provide protection to the charging circuitry, possibly leading to damage to the charging circuitry or another unsafe condition. Furthermore, the transient voltage suppressor may have been improperly connected to the charging circuitry during manufacture, or not connected at all.
Therefore, if an over voltage condition exists and the transient voltage suppressor is disabled or disconnected due to the over voltage condition or some other fault condition, the charging circuitry may be damaged. In addition, it is difficult to ensure that over voltage protection components, such as transient voltage suppressors, are operating correctly and properly attached to the charging circuitry. If the over voltage protection component is not operating correctly or attached to the charging circuitry effectively during manufacturing, the over voltage protection component will not even provide protection for a first over voltage fault. Accordingly, what is needed is a mechanism to ensure that circuitry is safe during two simultaneous faults, and to ensure that over voltage protection components are functioning properly after manufacture.